The present invention relates to a semiconductor device which is resistant to alternating thermal stresses and in which a semiconductor wafer, which is provided with at least one pn-junction and with solderable contacting coatings on its opposite surfaces, is permanently connected by means of a soft solder in direct planar contact with the current conducting terminals.
Semiconductor devices are known, such as rectifier elements or transistors, in which the semiconductor wafer, which has been pretreated in a diffusion process, is provided with nickel coatings on both major surfaces and with at least one further metal layer which enhances the soft soldering process, and this wafer is contacted by soft soldering these contacting coatings with housing portions and/or with connecting conductor parts, which are preferably made of copper, and is disposed in a housing. Solders on a basis of tin or lead with the addition of silver, indium, bismuth or antimony are provided as soft solders. Such soft solders, for example, are solders consisting of about 96% tin, about 4% silver and small proportions of bismuth or antimony, or of 92.5% lead with the remainder being tin and silver, or of 99% lead and 1% tin. As has been shown in tests, however, semiconductor devices for medium and high current carrying capabilities with contacting layers of such solders are not capable of withstanding use under stress with frequent changes in operating temperature since these extreme stresses cause the soft solder layers to exhibit fatigue after a certain period of time, which fatigue leads to the connecting conductor parts coming loose and thus cause the device to fail.
The difficulties encountered with such use of the above-mentioned devices when they are provided with soft solder contacts arise from the fact that the soft solders have significant physical property values, particularly their thermal expansion coefficient, their electrical and thermal conductivity and their tensile strength and yield strength, which deviate substantially from the corresponding values of the materials of the adjacent components.
Various proposals for solutions have been made to overcome these difficulties. Embodiments of semiconductor devices are known in which at least one disc of a high melting point metal, preferably tungsten or molybdenum, is hard soldered to the contacting surface of each of the metallic contact components which are intended to be connected with the semiconductor wafer, and the semiconductor wafer is fastened between these metal discs by means of a soft solder, so that the soft solder layer comes to lie between materials which have approximately the same coefficient of thermal expansion.
In other known embodiments such metal discs consist of layers of different materials, i.e., a layer of tungsten or molybdenum faces the semiconductor wafer and a layer of copper or silver faces toward the connecting conductor so that the coefficients of thermal expansion of adjacent materials approximately coincide. These metal discs are permanently connected, by means of soft solder, with the semiconductor wafer as well as with the connecting conductor parts.
Such known arrangements exhibit better resistances to alternating temperatures than arrangements without the above-mentioned metal discs. However, they have the significant drawbacks of high electrical and thermal resistance due to the thickness of the metal discs, which is considerable compared to the thickness of the soft solder layers, and of the need for considerable expenditures for the metals and the additionally required process steps. Furthermore the sintered discs developed in this connection, which discs comprise a plurality of metals, are expensive to manufacture and must be properly pretreated in order to be suitable for further processing.
Semiconductor devices with hard solder contacts are also known where such hard solder contacts are disposed between the semiconductor wafer, the metal disc and/or the connecting conductor. With such a structure the desired resistance to alternating temperatures is provided. However, under the high process temperatures required for the hard soldering process, undesirable reactions of the provided metals with the semiconductor material often substantially worsens the properties of the semiconductor material.
Semiconductor arrangements with high resistance to alternating temperature stresses are known in which semiconductor wafers are connected directly with connecting terminals in a planar manner to form conductive connections. For this purpose soft solders are used which have a lead or tin base and each contain small quantities of silver, copper or antimony. The resistance to alternating temperatures, however, is attained in these arrangements in that up to 50% of a high melting point metal in powder form is mixed with the soft solder and this metal will dissolve in the soft solder only incompletely or not at all at the soldering temperature required for the soft solder. The soft solder itself thus does not have the special property of being resistant to alternating temperature stresses and the substantial increase in surface area of the solder material produced by the addition of the metal powder results in a disadvantageous increase of the thermal transfer resistances of the soft solder layer.
Moreover eutectic gold/tin solders are also known which, as low melting point solders, do have the desired resistance to changes in temperature but which, due to their high gold content, constitute an undesirable cost factor in the assembly of semiconductor devices.